At present, a lithium secondary battery is generally employed as an electric source for driving small electronic devices. The lithium secondary battery is expected not only for the use as a potable electronic/communication tool such as small size video camera, potable phone, and note-size personal computer but also an electric source of automobile. The lithium secondary battery essentially comprises a positive electrode, a non-aqueous electrolyte solution, separator, and a negative electrode. A lithium secondary battery utilizing a positive electrode of lithium compound oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal is generally used. As the electrolyte solution, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is generally used.
When the lithium secondary battery is overcharged to a level higher than the ordinary working voltage, an excessive amount of lithium is released from the positive electrode, and simultaneously excessive lithium deposits on the negative electrode, and dendrite is produced. Therefore, both of the positive electrode and the negative electrode are rendered chemically unstable. If both of the positive and negative electrodes become chemically unstable, they soon react with carbonate in the non-aqueous electrolyte solution to decompose the carbonate, and sudden exothermic reaction occurs. Accordingly, the battery as such generates abnormal heat, and trouble of lowering of battery safety occurs. The trouble will be more serious in the case that the energy density of a lithium secondary battery increases.
Japanese Patent Provisional Publication 7-302614, for instance, proposes that a small amount of an aromatic compound be added to the electrolyte solution so that the safety to the overcharging can be ensured and the above-described trouble can be obviated. The Japanese Patent Provisional Publication 7-302614 describes anisole derivatives that have a molecular weight of not more than 500 and a π-electron orbital showing a reversible oxidation-reduction potential at a potential of more noble than the positive electrode potential in the case of full charging. It is explained that the anisole derivative functions as a redox shuttle in the battery so as to ensure safety of battery when the battery is overcharged.
Japanese Patent Provisional Publication 9-106835 discloses a method for ensuring safety of a battery under overcharging condition by employing carbonaceous material as the negative electrode and incorporating approx. 1 to 4% of an additive such as biphenyl, 3-R-thiophene, 3-chlorothiophene or furan into the electrolyte solution so that biphenyl or the like produces a polymer to enhance internal resistance of the battery when the voltage of the battery exceeds the predetermined maximum working voltage.
Japanese Patent Provisional Publication 9-171840 discloses a method for ensuring safety of a battery under overcharging condition by similarly employing biphenyl, 3-R-thiophene, 3-chlorothiophene or furan which polymerizes to produce gaseous material so as to initiate the internal current-disconnecting apparatus for forming internal short-circuit when the voltage of the battery exceeds the predetermined maximum working voltage.
Japanese Patent Provisional Publication 10-321258 discloses a method for ensuring safety of a battery under overcharging condition by similarly employing biphenyl, 3-R-thiophene, 3-chlorothiophene or furan which polymerizes to produce an electro-conductive polymer for forming internal short-circuit when the voltage of the battery exceeds the predetermined maximum working voltage.
Japanese Patent Provisional Publication 11-162512 points out a problem in the use of an additive such as biphenyl or the like in a battery in that the battery characteristics such as cycle characteristic are apt to lower when the cyclic procedure is repeated up to a voltage exceeding 4.1 V or the battery is discharged at a high temperature exceeding 40° C. for a long period of time, and that this problem is more prominently observed when the addition amount of additive increases. For ensuring a battery under over-charging condition, this publication then proposes an electrolyte solution in which 2,2-diphenylpropane or other additive is incorporated and the 2,2-diphenylpropane or the like polymerizes to produce a gaseous material to initiate the internal current-disconnecting apparatus or give an electro-conductive polymer for forming the internal short-circuit when the voltage of the battery exceeds the predetermined maximum working voltage.
Although the anisole derivative disclosed in Japanese Patent Provisional Publication 7-302614 favorably functions by redox shuttle in the case of overcharging, it has problems in that adverse effects are observed on cycle characteristics and storage stability. In more detail, the anisole derivative described in the publication gradually decomposes in the charge-discharge procedure when the battery is employed at a high temperature such as higher than 40° C. or subjected locally to a relatively high voltage in the use at an ordinary working voltage. Therefore, the battery characteristics lower. Thus, the amount of an anisole derivative gradually decreases in the course of ordinary charge-discharge procedures, and hence the safety may not be ensured after the charge-discharge procedures of 300 cycles.
The biphenyl, 3-R-thiophene, 3-chlorothiophene, and furan described in Japanese Patent Provisional Publications 9-106835, 9-171840, and 10-321258 also favorably work when overcharging occurs. However, as is pointed out in the aforementioned Japanese Patent Provisional Publication 11-162512, they impart adverse effect to the cycle characteristics and storage stability. Further, the adverse effect increases when the amount of biphenyl and the like is increased. In more detail, the biphenyl or the like is oxidized and decomposes at a potential of 4.5 V or less. Therefore, the biphenyl or the like decomposes in the charge-discharge procedure when the battery is employed at a high temperature such as higher than 40° C. or subjected locally to a relatively high voltage in the use at an ordinary working voltage. Therefore, the battery characteristics lower. Thus, the amount of a biphenyl or the like gradually decreases in the course of ordinary charge-discharge procedures, and hence the safety may not be ensured after the charge-discharge procedures of 300 cycles.
The battery containing 2,2-diphenylpropane which is described in Japanese Patent Provisional Publication 11-162512 does not show such a high safety at the time of overcharging as the safety shown in the battery containing biphenyl. However, it still show a high safety at the time of overcharging, as compared with a battery containing no additive. Further, although the battery containing 2,2-diphenylpropane shows high cycle characteristics as compared with the cycle characteristics shown in the battery containing biphenyl, it still does not show such high cycle characteristics, as compared with a battery containing no additive. Thus, the publication describes that the good cycle characteristics may be accomplished only when the safety is partially ignored. In consequence, it does not satisfy either battery characteristics or safety such as prevention of overcharging.
The invention has an object to provide a lithium secondary battery which are free from the above-mentioned problems, that is, which is improved in safety such as prevention of sudden heat generation at the time of overcharging, and other battery characteristics such as cycle characteristics, electric capacity and storage stability.